1. Field of the Invention
The invention relates to control valves, and more particularly to control valve assemblies having separate intake and exhaust valves controlled by a cam.
2. State of the Prior Art
Air suspension systems have become increasingly popular for use in vehicle suspensions, seats, and cabs on semi-trailer truck rigs and other vehicles. Each requires a height control valve which maintains the design height of the suspension. In a semi-trailer suspension the design height is the spacing between the frame and the axle. The height control valve senses when the spacing is greater or less than its design value, and accordingly adjusts the pressure in the air springs disposed between the frame and axle. The air suspension system with a height control valve maintains a uniform height of the air spring over a range of vehicle loading conditions.
Some height control valves immediately respond to changes in vehicle height as the vehicle moves. But other height control valves are designed not to respond to momentary changes in the design height. A time delay mechanism in the valve delays actuation of the valve for a specified time after the valve senses a change in height, preventing the valve from reacting to temporary height changes. A dead band in the height sensing portion of the valve prevents the valve from attempting to correct every minute change in height.
Previous height control valves employed somewhat complicated and bulky time delay and dead band mechanisms. For instance, U.S. Pat. No. 3,884,454 issued May 20, 1975 to Passmore discloses a height control valve having an attached vane damper in which a vane rotates in a fluid filled damping chamber in response to height changes. A filling or exhaust valve is actuated to adjust the air spring only after the vane has rotated a specified amount. As the vane rotates away from its rest position, incompressible damping fluid must pass through the small space between the end of the vane and the chamber wall, slowing the motion of the vane. As the vane returns to its center a check valve opens to allow the fluid to quickly flow through a bypass channel. The vane thus causes the valve to open slowly, yet close quickly. The vane damper works well but is bulky and somewhat complicated. Also, the vane damper is not an integral part of the valve actuator, adding to the overall bulk and complexity of the valve.
Other height control valves employ a double ended piston, slidably mounted on an actuator rod, and adapted for reciprocal movement within a double ended fluid filled damping chamber in response to rotational movement of a height sensing arm. As one end of the piston enters an end of the chamber from the piston's at rest position, the fluid is forced past the piston retarding the piston's movement. Flapper valves on the piston allow it to exit the end of the chamber and return to its rest position quickly. The forward portion of each end of the piston has a large clearance so that as the piston returns to the central at rest position, and an end of the piston starts to enter an end of the chamber, there is a sufficient gap to allow fluid to freely flow past the piston and quickly return to center. Only as the piston moves from the center position is its movement retarded.
The piston must move a specified distance along the actuator rod before it engages a projection thereon and actuates the air valve. Movement of the actuator rod in one direction allows air to fill the air spring, and movement of the actuator rod in the opposite direction allows air to exhaust from the air spring. The piston has many surfaces with close tolerances making manufacturing more difficult.